There is no specific Microsoft Lifecycle Support Policy for Microsoft Security Essentials. However, according to the FAQs Microsoft will offer Mainstream Support for either a minimum of 5 years from the date of a product’s general availability, or for 2 years after the successor product (N+1) is released (MSE initial release September 29, 2009) whichever is longer.

Also from FAQs,

5. If my program is currently supported under the Support Lifecycle policy, but my operating system is no longer supported, can I still receive support for the program? For example, can I receive support for Microsoft Office 2000 running on Microsoft Windows 98?
If the problem is specific to the program, Microsoft will provide support. If the problem is a result of the combination of the operating system and the program, that particular problem will not be supported.

Then again the MICROSOFT SOFTWARE LICENSE TERMS-MICROSOFT SECURITY ESSENTIALS states the following:

9.SUPPORT SERVICES. Because this software is “as is,” we may not provide support services for it.

In other words, as contributors like myself have come to learn, "Subject to change without notice" I doubt you will even get any "written in stone" answer come April 2014. Now, if you had asked about Windows Defender I can advise that Mainstream Support will end shortly on April 10, 2012.

I can't fault that answer, it covered the whole field. And unlike me and many others, you must be an accomplished explorer in Microsoftland. I've had many searches where I've just given up. Thank you for a very clear explanation of how things stand. I've also learned a new use of a word - deprecated - in respect of what category MS place Windows Defender. (I had to look that up.)